Commit message: MDEV-25636: Bug report: abortion in sql/sql_parse.cc:6294

The asserion failure was caused by this query

  select /*id=1*/ from t1
  where
   col= ( select /*id=2*/ from ... where corr_cond1
          union
          select /*id=4*/ from ... where corr_cond2)

Here,
- select with id=2 was correlated due to corr_cond1.
- select with id=4 was initially correlated due to corr_cond2, but then
  the optimizer optimized away the correlation, making the select with id=4
  uncorrelated.

However, since select with id=2 remained correlated, the execution had to
re-compute the whole UNION. When it tried to execute select with id=4, it
hit an assertion  (join buffer already free'd).

This is because select with id=4 has freed its execution structures after
it has been executed once. The select is uncorrelated, so it did not expect
it would need to be executed for the second time.

Fixed this by adding this logic in
st_select_lex::optimize_unflattened_subqueries():

  If a member of a UNION is correlated, mark all its members as
  correlated, so that they are prepared to be executed multiple times.
CVE: CVE-2021-46664
CWE: CWE-476
Vulnerability description: MariaDB through 10.5.9 allows an application crash in sub_select_postjoin_aggr for a NULL value of aggr.

Project: server

bool st_select_lex::optimize_unflattened_subqueries(bool const_only)
{
  SELECT_LEX_UNIT *next_unit= NULL;
  for (SELECT_LEX_UNIT *un= first_inner_unit();
       un;
       un= next_unit ? next_unit : un->next_unit())
  {
    Item_subselect *subquery_predicate= un->item;
    next_unit= NULL;

    if (subquery_predicate)
    {
      if (!subquery_predicate->fixed)
      {
	/*
	 This subquery was excluded as part of some expression so it is
	 invisible from all prepared expression.
       */
	next_unit= un->next_unit();
	un->exclude_level();
	if (next_unit)
	  continue;
	break;
      }
      if (subquery_predicate->substype() == Item_subselect::IN_SUBS)
      {
        Item_in_subselect *in_subs= (Item_in_subselect*) subquery_predicate;
        if (in_subs->is_jtbm_merged)
          continue;
      }

      if (const_only && !subquery_predicate->const_item())
      {
        /* Skip non-constant subqueries if the caller asked so. */
        continue;
      }

      bool empty_union_result= true;
      bool is_correlated_unit= false;
      bool first= true;
      bool union_plan_saved= false;
      /*
        If the subquery is a UNION, optimize all the subqueries in the UNION. If
        there is no UNION, then the loop will execute once for the subquery.
      */
      for (SELECT_LEX *sl= un->first_select(); sl; sl= sl->next_select())
      {
        JOIN *inner_join= sl->join;
        if (first)
          first= false;
        else
        {
          if (!union_plan_saved)
          {
            union_plan_saved= true;
            if (un->save_union_explain(un->thd->lex->explain))
              return true; /* Failure */
          }
        }
        if (!inner_join)
          continue;
        SELECT_LEX *save_select= un->thd->lex->current_select;
        ulonglong save_options;
        int res;
        /* We need only 1 row to determine existence */
        un->set_limit(un->global_parameters());
        un->thd->lex->current_select= sl;
        save_options= inner_join->select_options;
        if (options & SELECT_DESCRIBE)
        {
          /* Optimize the subquery in the context of EXPLAIN. */
          sl->set_explain_type(FALSE);
          sl->options|= SELECT_DESCRIBE;
          inner_join->select_options|= SELECT_DESCRIBE;
        }
        if ((res= inner_join->optimize()))
          return TRUE;
        if (!inner_join->cleaned)
          sl->update_used_tables();
        sl->update_correlated_cache();
        is_correlated_unit|= sl->is_correlated;
        inner_join->select_options= save_options;
        un->thd->lex->current_select= save_select;

        Explain_query *eq;
        if ((eq= inner_join->thd->lex->explain))
        {
          Explain_select *expl_sel;
          if ((expl_sel= eq->get_select(inner_join->select_lex->select_number)))
          {
            sl->set_explain_type(TRUE);
            expl_sel->select_type= sl->type;
          }
        }

        if (empty_union_result)
        {
          /*
            If at least one subquery in a union is non-empty, the UNION result
            is non-empty. If there is no UNION, the only subquery is non-empy.
          */
          empty_union_result= inner_join->empty_result();
        }
        if (res)
          return TRUE;
      }
      if (empty_union_result)
        subquery_predicate->no_rows_in_result();
      if (!is_correlated_unit)
        un->uncacheable&= ~UNCACHEABLE_DEPENDENT;
      subquery_predicate->is_correlated= is_correlated_unit;
    }
  }
  return FALSE;
}